1. Field of the Invention
The present invention relates to a replacement nozzle for a pressurized vessel, and, more particularly, the present invention relates to a two-piece replacement nozzle and welding techniques associated therewith.
2. Description of the Related Art
While the present invention may be used in a variety of industries, the environment of a pressurized water reactor (PWR) nuclear power plant will be discussed herein for illustrative purposes. A typical PWR plant includes (in part) a reactor vessel, steam generator, pressurizer, and a reactor coolant piping system, all of which operate under high pressure. Nozzles are attached to the vessels and/or piping for a number of purposes, such as for connecting piping and instrumentation, providing vents, and securing control element drive mechanisms and heater elements. A typical pressure vessel in the form of a pressurizer 10 is shown in FIG. 1. The pressurizer 10 includes nozzles 12 for vents, nozzles 14 for sampling liquid level or pressure sensing, a nozzle 16 for temperature measuring, and a number of bottom nozzles 18, 20 for heating elements. All of these nozzles are welded to the pressure vessel at the time of original manufacture.
As shown in FIG. 2, cladding 22, typically made of stainless steel, is welded to the interior of the pressurizer 10, which is made of carbon steel. The nozzle 16 shown in cross section in FIG. 2 is exemplary of the mentioned welded nozzles, which all pass through a hole or bore 24 in the pressure vessel 10 and which are structurally welded at the interior end 26 to the vessel 10 with a J-groove weld 28 along the interior opening to the bore 24. The diameter of nozzle 16 is slightly less than the diameter of bore 24, so that there is a small annular space 30 between the nozzle exterior and the wall of bore 24. In some applications the nozzles are fit tight to the bore, and in a control rod drive mechanism, they are installed with a shrink fit process. The nozzles may also be roll-expanded in the bore 24. The J-groove weld 28 also functions as a seal weld to seal the annular space 30. A reactor vessel similarly has nozzles represented by nozzle 16 in FIG. 2 welded thereto. Corresponding reactor vessel nozzles are located in the lower spherical head and allow instrumentation to be inserted into the reactor core. The piping of the reactor coolant system (not shown) also includes similar nozzles welded thereto. Further details of pressurizer vessels, reactor vessels, and coolant system piping, in particular, and nuclear power facilities, in general, are known to those of skill in the art.
Nozzle failures and leakage in nuclear power facilities is mainly due to SCC (stress corrosion cracking), which occurs on components having a susceptible material, high tensile stresses, high temperature, and which are in a corrosive environment, conditions which primarily exist on nozzle penetration in the pressurizer vessel, reactor coolant piping, and the reactor pressure vessel. Such failures are manifested by cracking. Such cracking occurs at the grain boundaries on the inside diameter of the nozzle material at or near the heat-affected zone of the weld and propagates radially outward through the thickness of the nozzle, which can eventually lead to small leakage of the reactor coolant supply. Failures have also occurred on stainless steel pressurizer nozzles.
As indicated, nozzles of these types have failed over time and have had to be replaced or repaired, as inspections on the nozzles and welds have revealed small indications or, in some cases, very minor leakage of primary coolant. One known repair method entails machining off the portion of the original nozzle protruding from the pressure vessel, attaching a weld pad to the vessel surrounding the nozzle location, and welding a one-piece replacement nozzle to the weld pad, the replacement nozzle having the same or similar dimensions as the original nozzle. The weld pad and pad-to-nozzle weld required by applicable code with this method are relatively large due to the size of the replacement nozzle. The problems inherent with the large structural weld joint geometries include a long schedule duration for welding during implementation, a potential for high personnel radiation exposure during implementation, and an increased risk of potential welding issues during implementation.